1. Field of the Invention
The present invention relates to an optical disk, and more particularly to, an optical disk which can be efficiently prevented from being damaged, by forming an inner circumferential area of the optical disk by using a semi-crystalline polymer.
2. Description of the Related Art
In general, an optical disk is classified into a compact disk for recording data such as voice or image signals according to a digital recording method, and a digital versatile disk using a short wavelength of red semiconductor laser and having a large recording capacity.
FIG. 1 is a plane view illustrating a general optical disk.
Referring to FIG. 1, a hole 10a is formed at the center of the optical disk 10, an inner circumferential area 11 on which a recording film is not formed is formed on the outer circumferential surface of the hole 10a, and an outer circumferential area 12 is formed on the outer circumferential surface of the inner circumferential area 11.
Normally, the optical disk 10 is cracked through carelessness of the user, or when the optical disk 10 is composed of polycarbonate that is an amorphous polymer.
Even if only 1/10 of yield stress is continuously applied to polycarbonate that is the amorphous polymer at the normal temperature, polycarbonate is damaged.
The optical disk 10 may be damaged as follows. When the optical disk 10 is injection-molded or handled by the user, minute scratches formed around the hole 10a of the optical disk 10 cause cracks. Such cracks are gradually grown by a craze phenomenon, to damage the optical disk 10.
According to the craze phenomenon, a chain structure inside the amorphous polymer is collapsed and cut, and cracks are gradually grown, thereby finally damaging the optical disk 10.
In addition, an optical disk drive over 10,000 rpm, namely, 52x has been normally used. In the case of 52x, as shown in FIG. 2, a maximum yield stress of the optical disk 10 reaches 4.315 MPa.
In the 52x optical disk drive, if the optical disk 10 is damaged, a maximum speed of fragments reaches 66 m/s. That is, the fragments of the optical disk 10 may injure the user through a front surface of a casing of the optical disk drive.
The optical disk 10 is damaged because its material, polycarbonate is the amorphous polymer. Here, the optical disk 10 is damaged due to a static fatigue (or static load) at the normal temperature.
In order to prevent the optical disk 10 from being damaged, a crystalline polymer or a semi-crystalline polymer such as a thermosetting resin must be used to form the optical disk 10. However, the crystalline polymer or the semi-crystalline polymer has inferior optical characteristics. Technically, the crystalline polymer or the semi-crystalline polymer cannot be employed as the basic material of the optical disk 10.
That is to say, polycarbonate that is the amorphous polymer has been used to form the optical disk 10 to improve optical characteristics. However, polycarbonate is easily cracked due to the static fatigue, and such cracks are grown to damage the optical disk 10.
A method for preventing an optical disk from being damaged has been disclosed under Korean Laid-Open Patent Application 2001-0091302.
According to Korean Laid-Open Patent Application 2001-0091302, a metal material is molded in or inserted into the inner circumferential surface of the optical disk. However, an elastic modulus of the metal material is much higher than that of polycarbonate. As a result, a very high stress, about 52.5 MPa of stress is concentrated around the metal material, and thus the optical disk is more easily damaged than the general optical disk.